This invention relates to an angular velocity sensor, and more particularly to a sensor capable of detecting multi-dimensional angular velocity components every respective axes.
In the automobile industry, machinery industry, and the like, there has been an increased demand for sensors capable of precisely detecting physical action such as acceleration or angular velocity. In general, an acceleration in an arbitrary direction and an angular velocity in an arbitrary rotational direction are applied to an object which carries out free movement in a three-dimensional space. For this reason, in order to precisely grasp movement of the object, it is necessary to independently detect acceleration components every respective coordinate axis directions and angular velocity components about respective coordinate axes in the XYZ three-dimensional coordinate system, respectively. Therefore, there has been an increased demand for multi-dimensional acceleration sensors or multi-axial angular velocity sensors which are compact and has high accuracy, and which can suppress the manufacturing cost.
Hitherto, multi-dimensional acceleration sensors of various types have been proposed. For example, in the International Publication No. WO88/08522 based on the Patent Cooperation Treaty (U.S. Pat. No. 4,967,605 and U.S. Pat. No. 5,182,515), the International Publication No. WO91/10118 based on the Patent Cooperation Treaty (U.S. patent appln. No. 07/761,771), the International Publication No. WO92/17759 based on the Patent Cooperation Treaty (U.S. patent appln. No. 07/952,753), and the like, there are disclosed acceleration sensors in which applied acceleration components are detected every respective coordinate axes directions. The feature of these acceleration sensors is that a plurality of resistance elements, electrostatic capacitance elements or piezoelectric elements are arranged at predetermined positions of a substrate having flexibility to detect applied acceleration components on the basis of changes in resistance values of the resistance elements, changes in capacitance values of the electrostatic capacitance elements or changes in voltages produced by the piezoelectric elements. A weight body is attached on the substrate having flexibility. When an acceleration is applied, a force is generated at the weight body so that bending occurs in the flexible substrate. By detecting this bending on the basis of the above-described changes in resistance values, capacitance values or voltage values, it is possible to obtain values of acceleration components in respective axial directions.
On the contrary, the description of the multi-axial angular velocity sensors can be hardly found in the literatures up to now, and such angular velocity sensors have not been put into practical use. Ordinarily, angular velocity sensors are utilized for detecting an angular velocity of a power shaft, etc. of a vehicle, and only have a function to detect an angular velocity about a specific single axis. In such cases of determining a rotational velocity of the power shaft, it is sufficient to use an one-dimensional angular velocity sensor. However, in order to detect angular velocity with respect to an object which carries out free movement in a three-dimensional space, it is necessary to independently detect angular velocity components about respective axes of the X-axis, the Y-axis and the Z-axis in the XYZ three-dimensional coordinate system. In order to detect angular velocity components about respective axes of the X-axis, the Y-axis and the Z-axis by using the one-dimensional angular velocity sensor conventionally utilized, it is necessary that three sets of such one-dimensional angular velocity sensors as described above are prepared to attach them in such specific directions to permit detection of angular velocity components about respective axes. For this reason, the structure as the entirety of the sensor becomes complicated, and the cost also becomes high.
Under these circumstances, the inventor of the present application has proposed a novel multi-axial angular velocity sensor which is compact and has high accuracy, and which can suppress the manufacturing cost. This novel sensor has been disclosed in the International Publication No. WO94/23272 based on the Patent Cooperation Treaty. Moreover, the inventor of the present application has disclosed several improved sensors thereof in the Japanese Patent Application No. 191081/1994, the Japanese Patent Application No. 225894/1994 and the Japanese Patent Application No. 258909/1994. In accordance with these novel sensors, it is possible to detect three-dimensional angular velocity components about respective axes. This utilizes the principle that when an object is oscillated in the Z-axis direction in the state where angular velocity .omega.x about the X-axis direction is applied, Coriolis force is applied in the Y-axis direction. For example, an a.c. voltage is applied to a particular piezoelectric element disposed on a flexible substrate to oscillate a weight body attached on the flexible substrate in the Z-axis direction. When the angular velocity .omega.x about the X-axis is applied, Coriolis force is exerted on the weight body in the Y-axis direction. Accordingly, the weight body is caused to undergo displacement in the Y-axis direction. When this displacement is detected by charges produced by another piezoelectric element, the applied angular velocity .omega.x can be indirectly detected.
In the above-described multi-axial angular velocity sensor, it is relatively easy to carry out angular velocity detection about two axes. For example, if Coriolis force applied in the Y-axis direction can be detected in the state where the weight body is oscillated in the Z-axis direction, it is possible to obtain an angular velocity .omega.x about the X-axis. In contrast, if Coriolis force applied in the X-axis direction can be detected, it is possible to obtain an angular velocity .omega.y about the Y-axis. Namely, under the state where the weight body is oscillated in the same Z-axis direction, it becomes possible to obtain both the angular velocity .omega.x about the X-axis and the angular velocity .omega.y about the Y-axis. However, under the state where the weight body is oscillated in the Z-axis direction, it is impossible to obtain an angular velocity .omega.z about the Z-axis. In order to obtain the angular velocity .omega.z about the Z-axis, it is necessary to make a change to allow the weight body to be oscillated in the X-axis or Y-axis direction.
However, in order to change the oscillating direction of a weight body, which has a mass to some degree, from a first direction to a second direction perpendicular to the first direction, it is necessary to once stop the oscillation of the weight body thereafter to start to oscillate it in the second direction. Ordinarily, in order to stop the oscillating object, it takes time to some extent. In addition, in order to oscillate the object in a different direction to allow it to be brought into the stable oscillating state, it also takes a time to some extent. For this reason, in the case where detection of angular velocity about three axes is carried out, there was the problem that the response is lowered.